We are investigating mechanisms of bacterial drug resistance. In particular we are studying factors which influence resistance transfer in natural isolates of E. coli. For instance we are determining if lack of transfer in certain isolates is due to decreased mating-pair formation or to decreased transfer and establishment of the plasmid DNA. Factors which might influence these processes such as the presence of other plasmids or restriction endonucleases are being looked for. We found evidence for chromosomal control of pilus synthesis and we are studying which genes bring this about and what specific structures are being affected. We are also investigating the relationship between surface charge on bacterial cells and their capacity for entry exclusion of entering plasmids.